The invention relates to an integrated circuit comprising at least an inductive element, and a region called active region which may include resistive, capacitive and semiconductor elements, while the inductive element and part of the active region are superimposed.
A method for fabrication of such an integrated circuit is mentioned in U.S. Pat. No. 5,370,766. The object of this method is to reduce the total surface of the integrated circuit.
The invention is linked with the following considerations:
The superpositioning of an inductive element and the active region may cause the appearance of mutual inductances and parasitic couplings between the inductive element and the elements of the active region, which leads to a considerable deterioration of the quality factor of the inductive element and to a reduction of the precision of the frequency of the circuit.
A solution already known for minimizing the problems of interaction between the inductive element and the elements of the active region comprises putting the inductive element separate from other elements. Such an integration then poses a problem of bulkiness. Actually, to realize them, the inductive elements may require a quarter of the total surface of the circuit.
It is an object of the invention to largely remedy these drawbacks by proposing an integrated circuit in which at least an inductive element and an active region exist together between which the electromagnetic interactions are reduced without the bulkiness of the integrated circuit being increased significantly.
It is another object of the present invention to allow the integration of a compact integrated circuit which comprises at least an inductive element that presents a high quality factor.
Indeed, an integrated circuit in accordance with the opening paragraph is characterized according to the invention in that it comprises screening means for insulating the active region from an electromagnetic field that the inductive element is intended to develop.
In such an integrated circuit, the electromagnetic field which may be created by the inductive element is largely blocked by the screening means and its interaction with the elements of the active region is limited.
In an embodiment of the invention the screening means are placed between the inductive element and the active region and form an open circuit.
The object of this embodiment of the invention is to prevent screening means developing a mutual inductance with the inductive element. In this embodiment the screening means forming an open circuit cannot be passed through by a current induced by the magnetic field generated by the inductive element. Consequently, the mutual inductance existing between these screening means and the inductive element is very small.
In an advantageous embodiment of the invention, the screening means comprise a plate of low-resistive material placed perpendicularly to the vectors of the magnetic field intended to be developed by the inductive element, and formed by an alternation of bands and slots perpendicular to a current that could be induced in the plate by the inductive element, the bands being connected to an open frame.
Because of these characteristic features, such a plate blocks the propagation of an electric field to the active region of the circuit and functions as an open circuit with respect to a current that could be induced there. The mutual inductance that could be generated with the inductive element is thus nearly zero.
In a preferred embodiment of the invention, the screening means further include a via hole of low-resistance material whose walls surround the inductive element completely, said via hole having at least one slot over its entire height.
The inductive element, when positioned in the neighborhood of another inductive element, creates a mutual inductance with this other inductive element. This mutual inductance has a tendency to deteriorate the quality factor of the inductive element. The via hole enables to limit the creation of such mutual inductance by limiting the magnetic interaction of the inductive element with any other inductive element present in the circuit. A slot is provided over the entire height of the via hole so as to prevent the formation of a current loop at the surface of the via hole.
In an advantageous embodiment of the invention, the plate and the via hole are together connected to a reference terminal having potential. It is an object of the invention to limit the capacitive couplings between the elements of the circuit. By connecting the via hole and the plate to each other and to a same potential, limited parasitic capacitances will be created between the various elements of the circuit.
As the integrated circuit is basically formed by a superposition of layers, each made of a low-resistance material, the walls of the via hole are formed by a stacking of tracks, each cut out in one of said layers around a perimeter defined by the surface of the inductive element, while said tracks are interconnected. This embodiment of the invention is simple and cost-saving. The implementation of the via hole by the use of existing layers does not increase the size of the circuit.
In a particular embodiment of the invention, the integrated circuit comprises two inductive elements, the two being connected between a supply terminal and a reference terminal having potential, and each formed by a turn, said turns having symmetrical and opposite directions of coil winding, the respective parts of said facing turns being those that are farthest away from the power supply terminal. The choice of the directions of the coil windings influences the value of the mutual inductance which is formed when a current passes through the windings and, consequently, influences the value of the quality factor. This choice is made so as to optimize the quality factor of the inductive element.
Each inductive element will advantageously be surrounded by a via hole as described above so as to maximize the reduction of the electromagnetic interactions among said elements.
The present invention may be utilized in any integrated circuit in which an inductive element exists side by side with other elements, whether they are capacitive, resistive or semiconductor elements. The integrated circuit may, for example, be an oscillator, an active charge mixer, or a filter. In one of its applications, the invention therefore also relates to an oscillator intended to deliver an output signal that has a frequency whose value depends on the value of a tuning voltage, characterized in that the oscillator is realized in the form of an integrated circuit in accordance with the invention, of which the active region includes at least a varicap diode connected to the inductive element and intended to be biased by means of the tuning voltage.
More generally, the present invention may advantageously be used in an apparatus for receiving radio signals. The invention therefore relates to a radio signal receiving apparatus comprising:
an antenna and filter system enabling to receive a radio signal whose frequency, called radio frequency, is selected within a given frequency range, and its transformation into an electronic signal called radio signal,
a local oscillator whose frequency called oscillation frequency can be tuned as a function of a tuning voltage, and
a mixer, intended to receive the radio signal and a signal coming from the local oscillator and to deliver an output signal having a fixed frequency and equal to the difference between the radio frequency and the oscillation frequency, and
a signal processing unit intended to use the output signal of the mixer, which apparatus is characterized in that the local oscillator is in accordance with the oscillator described earlier.